1. Field of the Invention
This invention relates to a mounting device of a lithoplate to be mounted onto a plate cylinder of an offset press, and more specifically, it relates to a mounting device such as a clamp, positioning (or register) device, or the like, of a lithoplate.
2. Description of the Related Art
As a base material of a lithoplate to be used for lithographic printing, in addition to a metal base such as aluminum to be used as a PS plate, there are paper, a synthetic resin film, or the like, to which a water-proof treatment has been performed. They are widely used in view of their advantages in simplicity and ease of preparation thereof and used for various applications depending on their properties.
A PS plate can effect printing of close to 10 times the number of printed sheets as compared with a paper plate, but it is expensive. Therefore, in recent years in the field of light printing in which the number of printed sheets is not great, a paper plate, or a film plate, has been widely used.
A paper plate, or a plastic plate, is weaker in body or strength as compared with that of a PS plate, and cannot be directly fitted to a plate cylinder as is the case with the PS plate. When these plates are fitted to a plate cylinder, a head edge and a tail edge of a lithoplate are fixed with a vise, and the plate is fixed on a plate cylinder by applying tension, but the strength of the base is weak so that there is a problem of stability. Even if they can be fitted directly on a plate cylinder, wrinkles or creases are likely caused thereon when positioning adjustment of the printing plate is carried out after fitting it on the plate cylinder. Thus, they cannot be used in practical applications.
In order to solve the above problems, a method has been proposed in which a paper plate or a plastic film (particularly a polyester film) plate is not directly fitted on a plate cylinder. Instead, a plate cylinder packing comprising a PS plate, etc. is first fitted on a plate cylinder, a paper plate or a plastic film plate is fitted thereon or adhered thereto, and the plate cylinder packing is moved whereby positioning adjustment of the lithoplate is realized.
There has also been known a method in which a lithoplate attachment plate, or a lithoplate register pin (hereinafter sometimes referred to as "a register pin"), and a lithoplate holding plate are provided onto an upper blade of a clamp at the side of holding to which plate cylinder packing is fitted. Such methods are disclosed in, for example, Japanese Utility Model Publication No. 11782/1994 and Japanese Utility Model Laid-Open Publication No. 19037/1991.
FIG. 11 is a partial sectional view of a plate cylinder of a printing machine relating to Japanese Utility Model Publication No. 11782/1994 and Japanese Utility Model Laid Open Publication No. 19037/1991. In FIG. 11, in a cutout (notch) portion of a plate cylinder 7, a clamp 31 at a head holding portion and a clamp 32 at an end holding portion are contained. Each of the clamps mainly comprise upper blades 3 and 23, lower blades 4 and 24 and cams 5 and 25, respectively. The upper blades 3 and 23 are opened or closed by rotating the cams, respectively.
Since a lithoplate 1, having weak body or strength as in a paper plate or a film plate, cannot be directly fitted on the plate cylinder 7 as mentioned above, a plate cylinder packing 6 constituted by a PS plate, etc. is previously held to the plate cylinder 7 by holding with both clamps 31 and 32 at the head holding portion and the end holding portion, and a lithoplate 1 such as a paper plate of a film plate is adhered with paste, double-sided adhering tape or the like onto the lithoplate to fit the plate on the plate cylinder 7 through the plate cylinder packing 6.
In the above publications, as a positioning standard for a lithoplate, a register pin 2 is provided to an upper blade 3, and fixed to the clamp 31 at the head holding portion. The register pin 2 is inserted into a pin hole 1c of the lithoplate 1, and the lithoplate 1 is adhered to the plate cylinder packing 6.
However, the above method involves the problems that positioning error is caused at an upper blade 3 of a clamp 31 when fitting and removal of the clamp 31 at the head holding portion are repeated for changing a plate cylinder packing 6 whereby deviation occurs at the register pin provided onto the upper blade 3 of the clamp 31.
Thus, in a lithoplate mounting device using the conventional plate cylinder packing, it is laborious and time consuming to position the lithoplate comprising a paper plate or a film plate when the lithoplates are frequently changed. In multicolored printing, particularly in color printing, positioning of lithoplates is extremely important and it is strongly desired to overcome the above problems.
Also, at a lithoplate fitting operation, there is a problem that a lithoplate 1 rises and comes off of the register pin when inserting the pin hole of the lithoplate onto the register pin or adhering to a plate cylinder packing after insertion. In the above prior art, a lithoplate holding plate 8a is provided to prevent the above problem, but it is insufficient for accelerating and simplifying the fitting and removing operation of the lithoplate at the time of changing thereof. Particularly, when removing the lithoplate, it is difficult to remove it from the register pin so that, in the case of a paper plate, it is possible to remove the paper plate therefrom by breaking the plate around the pin hole, but then the paper plate cannot be used again after removal thereof. In the case of a film plate, breakage of the plate around the pin hole is not easy and removal thereof requires much labor. Therefore, it has been desired to overcome these problems.
On the other hand, a direct lithoplate utilizing a silver complex diffusion transfer method according to a scanning type exposure system has been developed in recent years. This lithoplate uses a paper or a film with a thickness of about 100 .mu.m as a support in relation to a power output machine. In this case the body and strength are very weak as compared with the conventional paper plate or film plate. Thus, it has also been desired to develop a smooth and rapid mounting device for inserting a lithoplate comprising the conventional paper plate or film plate with a relatively thick plate as well as a thinner lithoplate.
Further, in the above prior art method, the tail edge of the lithoplate is in a tail free state (also referred to as unfixed state or free end). That is, a length of the plate in the printing direction, i.e., a length between the head and bottom direction is set so that the end of the lithoplate is within the region to which ink is applied. By employing the above constitution, an operation of holding the lithoplate can be simplified and the size of the lithoplate can be made small. When the size of the lithoplate is small, there are advantages of not only saving lithographic materials, but also in the resulting wide flexibility when selecting a size of an output device of a lithoplate.
However, when the tail edge has a tail tree configuration, "rising" or floating up of the lithoplate end cannot be completely prevented. Also, there is a possibility of causing a problem that the lithoplate becomes caught under an ink roller when a printing machine is required to be rotated in reverse direction while winding the lithoplate, e.g., changing the lithoplate in a multi-color printer. Further, there is a problem that ink accumulates due to a bump at the tail edge of the lithoplate at the region in which ink roller pressure is applied onto the plate cylinder.
As a method for preventing "rising" of the tail edge of the lithoplate, there is disclosed a method in Japanese Utility Model Laid-Open Publication No. 88738/1991 that a holding mechanism (a clamp plate and a holding plate) is provided on an upper blade of a vise at the tail edge of the plate cylinder. In this method, however, there is a problem that insertion of the lithoplate tail edge into the holding mechanism is a troublesome operation. Further, in this method, the size of the lithoplate should be longer in the top to bottom direction than the regular lithoplate which is held to the plate cylinder by using a vise. This means there is a problem that a greater amount of plate material is required as well as the lithoplate is out of the range of the regular size or output of general applications.